The present invention relates to an electrophotographic photoreceptor which comprises an electroconductive support and, provided thereon, a photosensitive layer containing titanium dioxide having high sensitivity in longer wavelength region.
In general, electrophotographic photoreceptor comprises an electroconductive support on which is formed a photosensitive layer comprising a dispersion of a photoconductive material and, if necessary, a sensitizer in a binder resin. For formation of images, there has been well known so-called CPC method which comprises uniformly charging the photosensitive layer, imagewise exposing the charged surface to form an electrostatic latent image and then developing the latent image with developer to directly form a toner image on the photoreceptor. As the photoconductive material, zinc oxide has been most commonly used.
Recently, formation of so-called pictorial images by electrophotographic copying method has been more and more desired. Thus, with increase in demand for making high-quality images, it has been conducted to use a high-performance photoconductive material for photosensitive layer, to carry out various signal processings by changing original image to electrical signal by optoelectric transducing means or to apply so-called digital color image recording method according to which laser beam is modulated based on the above signal-processed information and scanning exposure is carried out by this laser beam.
Noticing the fact that as a photoconductive material of electrophotographic photoreceptor, titanium dioxide is superior to zinc oxide in tone reproduction of image, whiteness and hiding power, the inventors have conducted research to electrophotographically produce color image excellent in tone reproduction, graininess and resolving power by forming electrostatic latent image using said titanium dioxide photoreceptor and then developing the latent image with a developer containig toner particles, especially a liquid developer excellent in image graininess. As a result, it has been found that quality of the resulting color image is markedly improved, but there still remain some problems to be solved.
Cyanine dye sensitizers used for enhancing sensitivity in the region from visible long wavelength to near infrared wavelength are not necessarily sufficient in adsorbability to titanium dioxide particles and these sensitizers are apt to be much damaged in stability when they are used for sensitization to the longer wavelength region and thus they cannot impart stably the desired spectral sensitivity for a long time. Especially when semiconductor laser is employed as a light source, scanning exposure with laser beam is conducted, being different from the conventional whole surface exposure with visible light, it is important that change in unexposed portion is sufficiently retained for the period of from beginning of exposure to termination of exposure. As a result of intensive research conducted by the inventors in an attempt to solve the above problems, it has been found that the above problems can be solved by a photoreceptor comprising titanium dioxide sensitive to longer wavelength light prepared by modifying the surface of titanium dioxide particles by previous treatment with a mineral acid and then allowing the surface of the particles to support a cyanine dye sensitizer and a hydrophobic organic compound. Thus, the present invention has been accomplished.